Page 215 - Microtectonics

7.5 · Millipede, Deflection-Fold and Oppositely Concave Microfold (OCM) Microstructures 205
7.5 lation cleavages. Bell (1981) claimed that deformation 7.5
Millipede, Deflection-Fold and in rocks is generally partitioned into lenses (‘pods’) with
Oppositely Concave Microfold (OCM) Microstructures little deformation or predominantly coaxial shortening,
surrounded by an anastomosing network of shear zones
Bell and Rubenach (1980) and Bell (1981) have drawn (Fig. 7.25). He envisaged that porphyroblasts with
attention to a microstructure in some inter- and syntec- millipede structures grow syntectonically in these pods
tonic porphyroblasts, which they named millipede micro- until they impinge on the surrounding shear zones where
structure (Fig. 7.22a, ×Video 7.22). The term refers to they stop growing or dissolve. This does not seem to
syn- or intertectonic porphyroblasts around which S is be generally valid, however; many millipede structures
e
deflected in opposite directions (Fig. 7.23, ×Photo 7.23). and deflection folds may be at least in part intertectonic,
In a related type of structure, S is deflected through iso- and may have grown over a straight foliation before a
e
clinal folding at both sides of a porphyroblast in a way as second foliation is developed; they should therefore be
shown in Fig. 7.22b. This structure has been called de- interpreted with care. The idea that porphyroblasts
flection-fold structure by Passchier and Speck (1994; stop growing when they reach a shear zone or cleavage
Fig. 7.24). Johnson and Moore (1996) introduced the con- lamella (Bell 1981) seems also not universally applicable
cept of oppositely concave microfolds (OCMs), referring as illustrated by a staurolite porphyroblast that over-
to similar structures. These structures are an effect of grew a cleavage lamella (Fig. 7.19b). In a review of five
foliation deflection adjacent to a rigid porphyroblast. different types of OCMs Johnson and Bell (1996) con-
Structures similar to deflection folds and millipede clude that these structures do in general not provide
structures can be reproduced experimentally in homo- unequivocal evidence for non-coaxial or coaxial defor-
geneous, non-partitioned flow around rigid objects mation histories. However, finite longitudinal strains can
(Ghosh and Ramberg 1976; Masuda and Ando 1988; be measured in rocks that contain porphyroblasts with
Gray and Busa 1994). A possible alternative explanation OCMs, by comparing the spacing between cleavage
for these structures is presented by Beaumont-Smith planes in and outside the porphyroblasts (Johnson and
(2001) as the progressive overgrowth of conjugate crenu- Williams 1998).

Fig. 7.23. Albite porphyroblast (light grey) with inclusion pattern characteristic of a millipede microstructure. S corresponds to S and S e
i
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to S . Garnet albite schist. Elephant Island, West Antarctica. Width of view 2.8 mm. CPL
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